Exhaust gas recirculation systems recirculate at least a portion of the gas exhausted generated by an internal combustion engine back into the engine. For certain conventional exhaust gas recirculation (EGR) systems, exhaust gas expelled from all of the cylinders of an internal combustion engine may be collected in an exhaust manifold. A fraction of the collected exhaust gas (e.g. 5% to 10%) may then be routed from the exhaust manifold through a control valve back to an intake manifold of the engine, where it may be introduced to a stream of fresh (ambient) intake air. The remaining fraction of exhaust gas in the exhaust manifold, rather than being recirculated and recycled, generally flows to a catalytic converter of the exhaust system and, after treatment therein, may be expelled to the atmosphere through the exhaust pipe.
EGR has a history of use in gasoline spark-ignition engines, and affects combustion in several ways. First, the combustion in the cylinders of the engine may be cooled by the presence of exhaust gas. That is, the recirculated exhaust gas may absorb heat from the combustion. Furthermore, the dilution of the oxygen present in the combustion chamber with the exhaust gas, in combination with the cooler combustion, may reduce the production of mono-nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide (NO2). Additionally, EGR may reduce the need for fuel enrichment at high loads in turbocharged engines and thereby improve fuel economy.
EGR, which uses higher levels of exhaust gas, may further increase fuel efficiency and reduce emissions of spark-ignition engines. However, with higher levels of exhaust gas, engines may face challenges related to EGR tolerance, which may reduce the expected fuel efficiency improvement. Challenges related to EGR tolerance may be understood to include increasing an engine's ability to process higher levels of exhaust gas without adversely affecting performance, particularly fuel economy. Thus, even if EGR tolerance may be satisfactory for engine operation at low levels of EGR, an engine may need additional modifications in structure and operational conditions to accommodate higher levels of EGR without adversely affecting engine performance.
More recently, an engine configuration has been proposed with one or more cylinders of the engine being dedicated to expelling exhaust gas for EGR, which is then directed to the intake manifold. Such cylinders may be referred to as dedicated EGR, or D-EGR, cylinders. Dedicated EGR cylinder(s) may operate at a broad range of equivalence ratios since their exhaust gas is generally not configured to exit the engine before flowing through a cylinder operating at, for example, a stoichiometric or near stoichiometric air/fuel ratio, at stoichiometric there is a correct amount of air and fuel such that, after combustion, all the fuel is burned and there is no excess air present. This may allow the dedicated EGR cylinder to be operated fuel rich to produce higher levels of hydrogen (H2) gas and carbon monoxide (CO) gas and which, may in turn, increase the octane number and promote increased EGR tolerance and knock tolerance by increasing flame/speed burn rates, as well as increasing the dilution limits of the mixture and associated combustion stability of all the cylinders. If the main cylinders are operated at an equivalence ratio of 1.0, stoichiometric exhaust leaves the engine and a conventional three way catalyst can be used to reduce pollutant emission.
There are a number of air intake and exhaust gas recirculation configurations for dedicated exhaust systems. For example, in some embodiments, recirculated exhaust gas may be intermixed with the intake air and supplied to all of the combustion cylinders, including the dedicated exhaust gas cylinder. In other embodiments, the dedicated exhaust gas cylinder may receive only ambient air from the air intake and the remainder of the cylinders (i.e. the main cylinders) may receive recirculated exhaust gas intermixed with intake air.
Due to the different air intake and exhaust gas recirculation loop configurations of dedicated exhaust gas recirculation (D-EGR) engines and the rich operating strategy, wherein the air to fuel ratio of the D-EGR cylinder may be in the range of 14.0 to 6, combustion behavior can vary significantly between the D-EGR cylinder(s) and the main cylinder, which may run at an air to fuel ratio in the range of 9 to 25. At part load or other stability limited conditions, where e.g., the break mean effective pressure may be in the range of 0 to 10 bar, the coefficient of variation of the indicated mean effective pressure is greater than 3 to 5%, or knock intensity is greater than 0.3 to 1.5 depending on engine configuration, the additional fuel dilution can cause unstable dedicated exhaust gas recirculation operation. At high loads or other knock limited conditions, the additional fuel dilution decreases the combustion temperatures and typically improves the knock resistance of the D-EGR cylinder(s) over the main cylinders. To balance the indicated mean effective pressure (IMEP) and torque fluctuations across all cylinders, individual spark timing, start of injection, valve timing and other methodologies that affect volumetric efficiencies (VE) of each cylinder, i.e., individual volumetric efficiencies, can be employed. The methodologies that enable indicated mean effective pressure balancing of the cylinders include, but are not limited to, individually dedicated exhaust gas recirculation cylinder(s) cam phasing and variable valve lift, a separate dedicated exhaust gas recirculation cylinder(s) intake port throttle, and a separate dedicated exhaust gas recirculation cylinder(s) exhaust port throttle.